The present invention relates to an easy to open package, to the film used in said package and to the polymer blend making up one of its layers.
In packaging there is a need for films which can be used, either alone or in combination with another film, to securely package articles and at the same time allow the easy openability of the package.
A wide variety of products, especially food products like meat, sausages, cheese and the like, are being offered in visually attractive packages made from two thermoplastic webs using vacuum skin packaging, thermoforming or other processes.
In a vacuum skin packaging process, a product is placed on a support, either in the form of a flat sheet or of a shaped tray, and, by differential air pressure, a heated film above the product is molded down, upon and around the product, and against the support, the space between the upper film and the support having been evacuated. The upper heated film thus forms a tight skin around the product and is sealed to the support.
In the thermoforming process a product is placed on the support, and the upper thermoplastic web is sealed to the flange-like edges of the support using heated sealing bars or similar equipment.
In both cases the two thermoplastic webs are sealed together, and safely secure the product therebetween. Opening of the packages thus obtained may be a problem as it may require the use of scissors, knives or other cutting and dangerous devices.
To prevent this problem the packages should be easily openable by manually pulling apart the two webs, normally starting from a point like a comer of the package where the upper web has purposely not been sealed to the support. Packages that can be opened in this way are called xe2x80x9ceasy-to-openxe2x80x9d or xe2x80x9cEZOxe2x80x9d packages.
The easy-to-open packages of the prior art are based on three different mechanisms.
In the first one (xe2x80x9cpeelable easy openingxe2x80x9d) the package is opened by separating the two webs at the seal interface. This concept is being widely used in thermoformed, form-fill-seal and also in vacuum skin packages. The easy opening or peeling strength (force per given width) is identical to the seal strength and may be controlled by an appropriate choice of the chemical similarity or dissimilarity of the sealing layers of the upper and lower webs. It is a disadvantage of this type of easy opening mechanism that it is strongly affected by the conditions of the sealing process and particularly by even minor variations in the sealing temperature and pressure Therefore the easy opening strength will differ widely for a series of packages and a rupture of the web that is torn to open the package (typically the upper web) may occur when it is sealed too strongly to the other one (the lower web).
In the second mechanism (xe2x80x9cadhesive failurexe2x80x9d) the opening of the package is achieved through an initial breakage through the thickness of one of the sealing layers followed by delamination of this layer from the underlying support or film. A typical example is a system where the seal layers of both the upper and lower webs are made from polyethylene and one of the seal layers is adhered to a polyamide surface. The low bond between polyethylene and polyamide permits the delamination to take place during the opening of the package. When the delamination reaches the area of the packed products, a second breakage through the sealing layer takes place. As a result the entire sealing layer of one of the two webs is separated from one of the webs and is left sealed to the opposite web.
The easy opening strength in this case is directly related to the bond between the seal layer and the adjacent film layer and accordingly it depends primarily on the chemical similarity or dissimilarity of the two materials. However, the coextrusion conditions such as pressure, temperature and time of contact between the molten materials also have a major effect on the final bond strength between the two layers.
The third system is based on the xe2x80x9ccohesive failurexe2x80x9d mechanism. In this latter system the easy opening feature is achieved by internal rupture of a seal layer that, during opening of the package, breaks along a plane parallel to the layer itself.
An example of easy-to-open packages based on this mechanism is provided in EP-B-192,131 describing an easy-to-open package with a seal layer comprising
a) an ionomer having a melt flow index of less than 5, and
b) a modified ethylene/vinyl acetate copolymer having a substantially higher melt flow index, whereby the melt flow indices of the two polymers in the seal layer differ by at least 10.
This system is based on the different chemical nature of the two components. The two materials are only partially compatible and consequently the material fails when a transversal force is applied to this layer (packaging opening) The easy opening system described in EP-B-192,131 shows however a high variability in the easy opening strength, or % variation (3"sgr") from the average value, that is typically higher than 55% and up to about 120%. Furthermore the average value of the opening strength generally obtained is  greater than 6.00 N/25.4 mm. Said value is currently considered too high for an easy to open package, particularly if the seal area varies in a great range, such as in vacuum skin packaging.
The high variability is probably due to the fact that the compatibility of the blend described in EP-B-192,131 can be affected by even small variations in the method of manufacturing of the web, e.g. a higher or lower opening strength can be obtained by varying the extrusion temperature or the quenching conditions by few degrees C.
There is a need of an improved easy opening system assuring a lower easy opening strength and also a lower % variation (3"sgr") from the average value.
It has now been found that if the sealing layer of a thermoplastic web comprises a blend of
(i) a copolymer of ethylene and acrylic acid or methacrylic acid,
(ii) a modified EVA copolymer, and
(iii) a polybutylene,
said layer will cohesively fail and rupture internally even when a transversal force lower than 6.00 N/25.4 mm is applied.
It has also been found that the % variation (3"sgr") from the average value is remarkably reduced and is less than about 55%, typically less than about 35% of the average value.
Said thermoplastic web can therefore be employed in the manufacture of an easy-to-open package where an article is wrapped into the thermoplastic web or is placed between a lower and a upper thermoplastic web wherein at least one of said webs comprises a sealing layer comprising the above blend.
It has also been found that is possible to obtain an easy-to-open package even when the blend is not in the sealing layer but in a layer directly adhered to the sealing layer provided that said sealing layer has a thickness of less than 10 xcexcm, and preferably less than 7 xcexcm.
A first object of the present invention is therefore an extrudable blend of
(i) a copolymer of ethylene and acrylic acid or methacrylic acid,
(ii) a modified EVA copolymer, and
(iii) a polybutylene
A second object of the present invention is a thermoplastic film wherein the sealing layer, or the layer directly adhered to the sealing layer, comprises a blend of
(i) a copolymer of ethylene and acrylic acid or methacrylic acid,
(ii) a modified EVA copolymer, and
(iii) a polybutylene.
A third object of the present invention is an easy-to-open package comprising:
a lower thermoplastic web including a first sealing layer (a),
an upper thermoplastic web including a second sealing layer (b) and
optionally an additional thermoplastic layer (c) adjacent and directly bonded to one of the sealing layer (a) and (b),
the lower and upper webs being sealed to each other and enclosing an article, said package being characterized in that one of the layers (a), (b), or (c) comprises a ternary blend of
(i) a copolymer of ethylene and acrylic acid or methacrylic acid,
(ii) a modified EVA copolymer, and
(iii) a polybutylene.
A fourth object of the present invention is a method of forming an easy-to-open package, which method comprises disposing a product between a lower thermoplastic web including a first sealing layer (a) and an upper thermoplastic web including a second sealing layer (b), heating the upper web and moulding it down upon and around the product and against the support, the space between the heated upper film and the support having been evacuated to form a tight skin around the product, and sealing said upper film to the support by differential air pressure said method being characterized in that one of the sealing layers (a) and (b) or a layer (c) directly adhered to one of the sealing layers (a) and (b), comprises a ternary blend of
(i) a copolymer of ethylene and acrylic acid or methacrylic acid,
(ii) a modified EVA copolymer, and
(iii) a polybutylene.
In a preferred embodiment the layer comprising the blend is not a sealing layer, but a layer (c) directly adhered to one of the sealing layers, thus assuring a good seal strength and lowering the dependence of the easy opening strength from the process conditions.
In such a case the thickness of the seal layer to which said layer (c) is directly adhered will be less than 10 xcexcm, preferably less than 7 xcexcm.